The present invention relates to a thermal transfer recording apparatus.
Thermal transfer recording is defined as a recording system in which ink of a thermal transfer recording medium such as a ink ribbon or an ink sheet is transferred and recorded onto a image receiving medium such as a recording paper by the heat of a thermal recording head which will be merely called a head hereinafter. This recording system can be divided into two main classes of a serial system and line system. In the serial type of thermal transfer recording system, recording is conducted as follows: in the head, a plurality of heating elements, for example 24 heating elements, are arranged in the direction of recording paper feeding, wherein the length of the arranged heating elements is equal to the width of a line of printed characters; while the head is moved in the direction perpendicular to the recording paper feeding direction, a line of characters are printed on a recording paper through an ink sheet or ribbon; then the head is withdrawn to the initial position synchronously when the recording paper is moved to the position of the next line; and the next printing operation is started. In the line type of recording system, operations are conducted as follows: in the case that the head is not moved; in the head, a plurality of heating elements, for example 2000 heating elements, are arranged in a line in the direction perpendicular to the recording paper moving direction; and while the recording paper and ink sheet are being moved, printing of a line of characters is conducted dot by dot in the direction vertical to the recording paper moving direction.
The ink ribbon or ink sheet is usually provided in a cassette in such a manner that: it is wound around a supply reel or supply shaft; and the leading edge is mounted on a take-up reel or shaft, so that when the take-up reel or shaft is rotated, the ink ribbon or ink sheet travels.
There are two kinds of ink ribbon or ink sheet, one is a one-time ink ribbon or sheet which can be used only once, and the other is a pluraltime repeatable ink ribbon or sheet which can be used a plurality of times. The pluraltime repeatable ink ribbon or sheet can be repeatedly used until its availability is used up, so that it is convenient compared with the one-time of ink ribbon or ink sheet since the number of exchange of the exhausted ink ribbon or ink sheet can be reduced. However, there are following problems.
FIG. 10 is an enlarged sectional view of the head and the portion close to the head in the case of the line system.
The head 84 is composed as follows. A heating element layer 86 made of tantalum nitride is provided on an alumina base 91 supported by an aluminum base 92; a common electrode 87 is provided to one edge of the heating layer 86 in the direction vertical to the surface of the drawing; a plurality of individual electrodes 88 are provided to the other edge in the direction vertical to the surface of the drawing; and the surface of the head is covered by a protective layer 89 made of an abrasion-resistant material such as silicon nitride.
A heating portion 84a is formed in the position where the common electrode 87 and individual electrodes 88 are spaced and opposed to each other. When an electric signal is selectively impressed upon the individual electrodes 88 by an IC for driving use not shown in the drawing, the heating portion 84a is heated dot by dot, so that ink 80c is transferred from an ink sheet 80 onto a recording paper 85 dot by dot.
The ink sheet 80 is composed of an ink layer 80a and a film 80b made from polyethylene terephthalate (PET) which supports the ink layer 80a. The ink sheet 80 comes into contact with the head on the side of the film 80b. The recording paper 85 is contacted with the ink layer 80a on the ink sheet 80 and pressed against a platen 90A by the press force of the head 84. When the heating portion 84a is selectively heated, the ink layer 80a on the ink sheet 80 is transferred onto the recording paper 85 dot by dot. In the case of a pluraltime repeatable type of ink sheet, only a portion of the ink layer 80a comes off in the position where transfer is performed, so that a cut-out portion 80d is formed in the ink layer 80a.
In a conventional recording apparatus, printing density is controlled so that it can be kept constant in such a manner that: a thermometer is provided to the head; and energy supplied to the heating portion of the head is adjusted in accordance with the measured temperature. Otherwise, printing density is controlled in such a manner that: an information of the operation of turning on and off conducted in the previous dot and in the dot before previous one, is memorized and energy supplied to the heating portion of the head is adjusted in accordance with the stored history.
However, in the case of a pluraltime repeatable type of ink sheet, only a portion of ink is transferred onto a transfer paper in a one-time printing operation, so that the amount of ink transferred onto the paper depends on the energy supplied by the head as well as the temperature condition of a separating portion 22. Accordingly, it is impossible to obtain uniform printing quality by the conventional method in which energy supplied to the head is only controlled.